Make API tests using HUB_TESTS_REF variable

Adding HUB_TESTS_REF variable to make test-api target to (optionally) set
Hub API tests branch.

Related to konveyor/ci#22 and konveyor/ci#23

Signed-off-by: Marek Aufart <maufart@redhat.com>

Makefile

@@ -34,7 +34,7 @@ test-tier0:
 
 # TIER1 - all normal features expected to work.
 test-tier1:
-	${MAKE} test-hub-api
+	${MAKE} test-hub-api ${HUB_TESTS_REF}
 	$(MAKE) test-metrics
 	TIER1=1 $(MAKE) test-analysis